This invention relates to a thrust ball bearing mounted between two members that make an eccentric revolving motion relative to each other, such as a stationary scroll and a revolving scroll of a scroll compressor.
A thrust ball bearing used in a scroll compressor is disclosed in Japanese patent publication 10-89350. As shown in FIGS. 4A and 4B, the thrust bearing comprises an opposed pair of annular races 1 and 2 having a plurality of opposed pairs of annular raceways 3 and 4 arranged at predetermined angular intervals, and balls 5 having a diameter dw (FIG. 4A) and each disposed between one of the pairs of raceways 3 and 4. The balls 5 are made of a bearing steel.
The race 1 is mounted to a frame 8 secured to a stationary scroll 7 and the race 2 is mounted to a revolving scroll 6.
The scrolls 6 and 7 have spiral partitioning walls 9 and 10, respectively, each received in the space defined by the walls 10 and 9 of the mating scroll. A compression chamber 11 is defined between each pair of the adjacent partitioning walls 9, 10.
The revolving scroll 6 is supported through a bearing on an eccentric rotary shaft 12 at its portion offset from its center of rotation by a distance e so that the scroll 6 revolves around the center of rotation of the shaft 12 without rotating about its own central axis. Each raceway 3, 4 has a groove bottom pitch circle diameter d that is set to be substantially equal to the amount of offset e to obtain a predetermined compression ratio in the compression chamber 11. The races 1, 2 are usually formed by pressing.
The thrust ball bearing bears a thrust load accompanying compression and also prevents the revolving scroll 6 from rotating about its own axis.
FIG. 5 is an enlarged view of the race 1. The annular raceway 3 comprises an annular groove 13 having an outer edge 14 and an inner edge 14xe2x80x2, an outer shoulder 15 and a flat portion 17 formed outside the outer edge 14, and a central land 16 formed inside the inner edge 14xe2x80x2. The central land 16 comprises a top and an inner shoulder 18 disposed between the top and the inner edge 14xe2x80x2. The central land 16 is symmetrical in section with respect to its top, and its top is substantially at the same level as the flat portion 17. The outer and inner edges 14 and 14xe2x80x2 are also at the same level a from the groove bottom.
The groove bottom pitch circle diameter d of a bearing of this type is usually set to be smaller than the ball diameter dw. For compactness and life of the bearing, the ratio dw/d should be as large as possible. The ball diameter dw is determined in view of durability of the balls. Thus, it is necessary to reduce the value d as much as possible while keeping the compression ratio at a predetermined value.
If the value d is reduced too much, the central land 16 would be too low in design and from a manufacturing viewpoint. If the central land 16 is too low, the contact ellipse A or contact impression might ride onto the shoulder 18 of the central land 16 as shown at B of FIG. 5. This increases surface pressure and shortens the life of the bearing.
The groove bottom pitch circle diameter d is set to a value as small as possible so long as it will not result in decrease in the life due to low accuracy in forming the groove. Based on the diameter d thus determined, the height a of the inner edge is then determined to a suitable value.
The contact ellipse is a contact impression having an elliptical shape and formed by contact of the ball 5 with the raceway 3, 4. For longer life of the bearing, the raceway 3 has to be designed such that the elliptical contact impression would not extend to or ride onto the shoulder 15 or 18.
One way to solve this problem while keeping the diameter d small is to set the diameter d smaller than the amount of offset e. This would cause the ball 5 to roll near the outer shoulder 15 away from the inner shoulder 18.
But this solution has a drawback in that the elliptical contact impression might extend to or ride onto the outer shoulder 15 (as shown at C of FIG. 5). This also results in increased contact pressure and shortens the bearing life.
The same is true with the raceway 2.
An object of the invention is to provide a raceway design which prevents the elliptical impression from extending to the outer shoulder of the groove while keeping the diameter d smaller than the amount of offset e.
According to the invention, there is provided a thrust ball bearing comprising a pair of races that make an eccentric revolving motion relative to each other, the races forward with a plurality of opposed pairs of annular raceways, balls each received in an opposed pair of the raceways, each of the raceways comprising a groove having a groove bottom pitch circle diameter that is smaller than the amount of offset of the eccentric revolving motion, the groove having an inner edge and an outer edge, the height of the inner edge from the bottom of the groove being set at a predetermined value, the height of the outer edge from the bottom of the groove being larger than the height of the inner edge from the bottom of the groove.
With this arrangement, since the height of the outer edge from the bottom of the groove is set to be larger than the height of the inner edge from the bottom of the groove, the tendency of the contact ellipse to move outwardly beyond the outer edge under load can be suppressed even though the ball tends to roll outside of the groove bottom pitch circle.
According to this invention, the height of the outer edge is set at such a value that the contact ellipse of the ball would not come out of the groove beyond the outer edge.
According to this invention, each of the raceways further comprises a central land formed inside the inner edge and having a top and an inner shoulder provided inside the inner edge and connecting the inner edge to the top, an outer shoulder provided outside the outer edge, and a flat surface provided outside the outer shoulder, the raceways each being symmetrical in section with respect to the central land, the flat surface being at a higher level than the top of the central land.
According to this invention, the inner and outer edges are inflection points between the groove and the inner shoulder and between the groove and the outer shoulder, respectively.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: